This in vitro study examined the effect of concentration and/or incubation time of tannic acid to dentin and bacteria. The results showed that tannic acid inhibited the dissolution of dentin collagen treated with tripsin and phosphoric acid, depending on the increasing of concentration and incubation time of tannic acid. SEM study also revealed tannnic acid protected the collagen structure treated with tripsin and phosphoric acid. Minimal inhibitory concentration of tannic acid to S.mutans was 300 mug/ml, while that of another streptococci was 400mug/ml. Phenol coefficient of tannic acid to S.mutans OMZ 175 was 0.6.these results indicate that antimicrobial action of tannic acid give play to strain such as S.mutans. Strerillizing power of power of 300 mug/ml tannic acid with S.mutans OMZ 175 for 15 min incubation times was the highest among the strains which were haevested from stationally phase, survival rate of S.mutans OMZ 175 decreased to 2% of control. As the results of their being coated and/or uncoated saliva, the surface hydrophobicity of all restoratives and teeth decreased and their negative surface charge increased when these surfaces were treated with tannnic acid. The number of adherent streptococci decreased under above conditions. Antimicribial action of bonding agent including tannic acid showed higher than that of bonding agent without tannic acid. However the specimens without tannic acid which were not cured or were cred and immediately used investigation in early time also showed antimicribial action. Bonding agent including tannic acid exhibited high adaptation to enamel and dentin surfaces. As suggested these results that tannic acid improve anti-enzyme property of dentin and antimicrob ial action, the existence of tannic acid between tooth and restorative surfacies is useful for inhibitation of secondory caries.